1. Field of the Invention
The present invention relates to a compound semiconductor substrate used preferably for a high speed or high breakdown voltage semiconductor device etc.
2. Description of the Related Art
A semiconductor device using a nitride semiconductor represented by gallium nitride in hexagonal crystal form (GaN), aluminum nitride in hexagonal crystal form (AlN), etc., allows a high breakdown voltage and a high frequency, surpasses properties of a silicon (Si) semiconductor device which is currently dominant, and is expected to be used for power devices etc.
As a method of fabricating the semiconductor device of this type and the compound semiconductor substrate used for its fabrication at low cost, it is known that a buffer area constituted by a plurality of AlGaN-based nitride semiconductor layers whose compositions are different is formed on a silicon (Si) or silicon carbide (SiC) single crystal substrate, and a nitride semiconductor active layer is formed on the layers of the buffer area.
For example, Japanese Patent Application Publication No. 2007-67077 (patent document 1) discloses that a plurality of first buffer layers composed of AlxGa1-xN (0.5≦x≦1) and a plurality of second buffer layers composed of AlyGa1-yN (0.01≦y≦0.2) are alternately stacked.
Further, Japanese Patent Application Publication No. 2008-171843 (patent document 2) discloses that a buffer layer is formed having a composite layer where a second layer formed using a nitride-based compound semiconductor in which an Al content is 0.8 or more is stacked on a first layer formed using a nitride-based compound semiconductor in which an Al content is 0.2 or less.
According to the buffer layer with the structure as disclosed in patent document 1 above, it is effective to form a nitride semiconductor which is flat and smooth without a crack. However, its breakdown voltage per unit film thickness is low and the compound semiconductor substrate suitable for a high breakdown voltage device may not be obtained. Further, it is not suitable for achieving a normally-off state.
On the other hand, patent document 2 discloses that it is possible to attain a high breakdown voltage by setting a carbon concentration of the first layer of the above-mentioned buffer layer as 1×1017 atoms/cm3 to 1×1020 atoms/cm3. However, even if the device is provided with the buffer layer having such a structure, it does not fully meet the recent demands for the high breakdown voltage and achieving the normally-off state.